Process and device for managing intercellular transfers of radio communications in a cellular radio communication system by measuring virtual speeds of cellular devices

ABSTRACT

A process for managing intercellular transfers of communication in a cellular radio communication system. A specified number of microcells inside umbrella cells each respectively possess a base station for allowing the routing of the communications. Radio direction finding is used to pinpoint at least two successive virtual positions of the mobile stations in communication in relation to the base station of the umbrella cell in order to deduce therefrom a speed of movement of each mobile station in the time interval which separates the pinpointing of their positions. The speeds of movement which are obtained are compared to the specified speed values so as to entrust the management of the communication from a mobile station to the base station of the umbrella cell in which it lies when its speed is greater than the specified speed value, or to the base station of the microcells when it is not greater than the specified speed value.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a process and a device for managingintercellular transfers of communications in a GSM-type cellular radiocommunication system.

2. Discussion of the Background

It applies in particular to the implementing of cellular radiocommunication systems in an urban environment in which the density oftraffic requires a multitude of cells and consequently a reduction intheir size.

In these systems, such as those described, for example in PatentApplications DE-A-4 414 428 and EP-A-0 589 278, the radio coverage of aterritory is achieved by using macrocells also termed umbrella cells,each of which encompasses a set of adjoining microcells, each cellpossessing a transmit and receive base station. The base station of anumbrella cell is generally arranged at a spot which is high up withrespect to the surface of the ground, on the roof of a building forexample, and its transmission power is defined so as to enable it tobroadcast information in a circular zone whose radius may reach severalkilometers. In a different way each transmit/receive base station of amicrocell services a zone of small radius which may reach a few hundredmeters, and their transmit/receive aerials are generally sited below thelevel of the roof of buildings, on public lampposts for example.

The umbrella cell makes it possible to absorb the excess trafficappearing in a micro-cell and to manage the gaps in coverage in themicro-cellular deployment. The microcells allow an increase in trafficin a given zone, and make it possible to compensate for coveragehindered by obstacles. However, the construction of microcells of smalldimensions gives rise to a complexity in the processing of the transfersof communications between microcells since the more the size of themicrocells is reduced, the more it becomes necessary to manage thetransfers between cells ever more frequently, and this is all the moreso the larger the speed of movement of the mobiles in communicationwithin the cellular network thus formed. This difficulty incurs risks ofcongestion of the multicellular network which takes a considerable timeto manage the signalling signals which accompany each transfer. Thisdifficulty is encountered in particular in GSM mobile radio networks inwhich no distinction is made between the mobiles in communication, apartfrom their power class, thus making it difficult to track mobiles incommunication so as to offload the communications of the fast mobilesonto the umbrella cell.

SUMMARY OF THE INVENTION

The purpose of the invention is to alleviate the aforesaid drawbacks.

For this purpose, the subject of the invention is a process for managingthe intercellular transfer of communications from mobile stations in acellular radio communication system comprising a specified number ofmicrocells inside umbrella cells each respectively possessing a basestation for allowing the routing of the communications, characterized inthat it consists, when the station is under the control of a basestation of a microcell, in counting the number of transfers H₀ performedbetween microcells by the mobile station during a specified observationinterval T₀, in calculating a rate of transfer H₀/T₀ for performing thetransfer of the communications to the umbrella station when the rate oftransfer is greater than a specified threshold value, in maintaining thetransfer of communications to the base station of the microcell when therate of transfer is below the specified threshold value and in that itconsists, when the mobile station is under the control of an umbrellacell, in using direction finding to measure a virtual speed of movementof the mobile station in relation to the base station of the umbrellacell in such a way as to perform a transfer of the communications to abase station of a microcell when the speed measured by the directionfinding is below a specified threshold value.

BRIEF DISCUSSION OF THE DRAWINGS

Other characteristics and advantages of the invention will emerge in thedescription which follows in conjunction with the appended drawingswhich represent:

FIG. 1, a cellular network structure according to the invention;

FIGS. 2 and 3, diagrams of the functional organization of a GSM network;

FIG. 4, a GSM network incorporating a radio direction finder accordingto the invention into an umbrella cell;

FIG. 5, an embodiment of a radio direction finder associated with a basestation of an umbrella cell for detecting the angle of arrival of themessage bursts transmitted by the mobile station.

FIG. 6, an illustration of a propagation of a multipath radio wavebetween a mobile station and a base station;

FIG. 7, an algorithm in the form of a flow chart for allocating themanagement of the transfers of the communications of the mobile stationsto the microcells or to the umbrella station.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The cellular network structure which is represented in FIG. 1 comprisesat least one umbrella macrocell 1 and a specified number of adjoiningmicrocells 2 arranged inside the microcell 1.

The microcells as well as the umbrella macrocell comprise, arranged attheir centre, a base station termed BTS, for transmitting radio waveswhose transmission power and radiation pattern delimit the zones ofterritory which are occupied respectively by the microcells 2 and theumbrella cell 1. In a GSM network this structure defines a cellularcommunication network which makes it possible to set up digitalcommunications between mobiles which are moving over the territoryoccupied by the various cells 1 and 2 of the network and subscribers ofthe switched public network. The communications are set up by virtue ofa series of functions which are those required in any mobile radiocommunication network, namely dialling, the routing of thecommunications to the mobile users, the transferring of communicationsbetween cells when the mobiles leave the territory occupied by a celland enter the territory of an adjacent cell etc. Conventionally thesefunctions are grouped together in functional units represented in FIG. 2and composed of mobile stations MS, of a radio subsystem BSS and of amanagement and routing subsystem NSS.

The mobile station MS allows the user to access the communicationservices offered. Each radio subsystem BSS caters for the communicationsover the whole of the territory delimited by a cells. Functionally, thissystem comprises a control function catered for by a base stationcontroller BSC and a radio transmission function supported by the basestations BTS of each cell. These latter stations manage the radio linkswith the mobile stations MS. The relationships between a base stationBTS and a base station controller BSC are defined by standardizedinterface circuits known by the name of “Abis”. The base stationcontrollers BSC are coupled to the remainder of the GSM network byinterfaces denoted A.

The management and routing subsystem NSS is composed of three elements,a mobile service switch MSC responsible for the task of routing thecommunications destined for the mobiles within a cells, a database HLRfor recording the permanent parameters of a subscriber and the locationof its mobile station, a database VLR in which a finer location of themobile stations in the call zone is recorded.

A typical configuration of dialogue between the stations MS, BTS and BSCis shown in FIG. 3. In this configuration the stations MS carry out adialogue via the previously indicated interfaces by using standardizedprotocols. The adapting according to the invention of the GSM network ofFIG. 3 to a network comprising microcells surrounded by umbrellamacrocells is achieved by incorporating, in the manner represented inFIG. 4, into the base station BTS of each umbrella cell a directionfinding station GS employing aerials situated at umimpeded spots abovebuildings. The base station controller BSC delegates it the task ofmanaging the traffic inside the umbrella cell using a new “Ater”interface. The controller BSC is in touch via the “Abis” standardizedinterface with the base stations BTS of the microcells. The basestations BTS of the microcells, are as in the case of FIG. 3,responsible for the task of managing the mobile stations MS lying intheir cell, provided however that they have a speed below a specifiedthreshold. The mobile stations which have a greater speed will bemanaged directly by the base station BTS of the umbrella macrocell.

The detection of a speed of movement of a mobile station is performedeither in a rough manner by counting within the macrocell, by means ofthe Abis interface, the number of transfers of communications betweencells when the mobile station leaves the territory of a microcell andpasses over to the territory of an adjacent microcell, i.e. moreprecisely by estimating the speed on the basis of a direction findingsighting performed by the radio direction finder of the umbrellastation. A radio direction finder capable of performing this sightingcan be the radio direction finder using the known antenna bearing thedesignation TRC 2966 and marketed by the Applicant. This radio directionfinder comprises in the manner represented in FIG. 5, a multichannelreception assembly 3 coupled to an antenna array 4 by way of switchingpreamplifiers 9, associated with a channel gauging generator 10. Themultichannel reception assembly 3 is synchronized with the BTS stationso as to allow the angular locating of the mobile stations MS whilebenefiting from the best signal-to-noise ratio. The assembly 3 receivesfrom the station BTS

a synchronization pip enabling it to open reception windows whereinshould lie the bursts of signals expected from the mobile stations;

a frequency control for positioning the multichannel receivers on thefrequencies of the mobile stations;

an indicator for signalling each type of burst expected;

and a code for indicating the sequence number to be searched for in thecase of a traffic burst.

On this basis a processor, not represented and belonging to thereception assembly 3 performs a search for synchronization with thebursts from sought-after mobile stations so as then to carry outdirection finding on these bursts, so as to determine for each mobilestation sought-after, the corresponding angle of arrival of the burstsof signals, the station BTS combines this information with the distanceseparating it from the umbrella base station so as to determine itsspeed of movement.

The search for synchronization is carried out in a known manner bycorrelating the reference sequence of each sought-after “burst” with thesignal received over a horizontal corresponding to the uncertainty ofreception of the GSM “bursts”, this typically being 8.25 symbols. Thissearch is performed conventionally in accordance with four steps. Thefirst step consists in estimating the correlation matrix for the signalsreceived on the N antennas of the array 4. Denoting by x(t) the vectorof the complex envelopes of the signals received on the N antennas thecorrelation matrix R_(x) can be written:

R _(x) =E[x(t)·x(t)⁺]  (1)

where E is a symbol which denotes mathematical expectation and x(t)⁺denotes the vector which is the conjugate transpose of x(t).

The second step consists in estimating a correlation vector Vxd(p) for acomplex signal d representing the reference sequence of the burst ofsignals received with each vector X of the complex envelopes of signalsreceived on the N antennas of the array 4 according to the relation:

Vxd(p)=Σx(n+p)·d(n)  (2)

where p represents the various possible positions of synchronization.

Estimation of the position of the synchronization is carried outaccording to a third step by calculating a vector V such that

V=Vxd*·R _(x) ⁻¹ ·Vxd  (3)

where Vxd is the vector conjugate to Vxd.

Finally, the fourth step consists in searching for the maximum value ofthe estimate of the synchronization which most precisely positions thereception channels on the most favourable signal-to-noise ratio.

The determination of the angle of arrival of the bursts of signals canbe carried out using various methods and in particular by using thealgorithm known by the acronym MUSIC, this being the abbreviation ofMultiple Signal Classification. A description of this algorithm can befound in particular by consulting the article by R. O. SCHMIDT entitled“Multiple Emitter Location and Signal parameter estimation system”published in the IEEE review of March 1986.

The algorithm breaks down into a calculation of a correlation matrix Rxxfor the signals applied to the N antennas of the radio direction finder,to a diagonalization and a decomposition into eigenvectors of the matrixRxx, and then into a detection of the minimum eigenvalues fordetermining the angle of arrival of the waves at the antenna array.

The determination of the distance is performed by determining the timeof propagation of the radio wave between the mobile station and the basestation of the umbrella macrocell.

Access to the GSM network is based on the principle of TDMAtime-division multiplexing. To do this, the base station BTS which is incharge of the time allots a time window to each mobile station, duringwhich the interrogated mobile station is supposed to respond. The startof the window is regarded as being the instant of arrival of theresponse from the mobile station when the latter is a distance of zerofrom the base station.

When the mobile station is far away from the base station, the responsefrom the mobile station occurs at a later instant shifted with respectto the start of the window. This shift determines the distance whichseparates the mobile station MS from the base station BTS. Thisprocedure which is dubbed “Timing advance” in the GSM standard serves toreturn to the mobile station MS the measured time shift so as toanticipate the transmission of the sequence of information bits by theduration of this shift so that the information which it transmits can bereceived in full within the time window allocated to it by the basestation.

By performing for example these measurements at a rate of twomeasurements per second, it is possible to compensate for the time shiftby the “Timing advance” procedure for speeds of movement of the mobilestations of up to 500 km/H.

Because of the fact that in an urban environment synchronization can beobtained on a path for propagating the radio waves by reflections on thefacades of buildings, the base station controller BSC determines in thiscase a virtual distance DV represented in FIG. 6, this being definedfrom the time shift TA expressed as a number of symbols, through therelation $\begin{matrix}{{DV} = {\frac{TA}{2}\left( \frac{48}{13} \right)10^{- 6} \times 310}} & (4)\end{matrix}$

where $\frac{48}{13}10^{- 6}$

represents the duration, in the GSM standard, of a GMSM modulationsymbol.

A virtual position of the mobile station is determined in relation tothe base station of the umbrella macrocell by combining the virtualdistance with the virtual angle of arrival of the radiowave through therelations

XY=DV cos (AV)  (5)

YV=DV sin (AV)  (6)

where AV denotes the virtual azimuth of the mobile station as determinedby executing the previously described MUSIC algorithm.

These Cartesian position elements are associated with each mobile.

They are used and combined so as to deduce therefrom their speeds ofvirtual movement.

As indicated previously, the exact position of the mobile with respectto the base station of the umbrella macrocell may not be determinedaccurately on account of the presence in an urban environment ofmultiple reflectors which give rise to multipaths. Under theseconditions only the distance which the mobile moves between twomeasurements appears to be sufficient. To obtain this path, thealgorithm used consists in taking two successive virtual positions ofthe same mobile XV₁, XV₂, and YV₁ and YV₂ respectively to calculate itsvirtual speed of movement (VDV) through the relation: $\begin{matrix}{{VDV} = \frac{\sqrt{\left( {{XV}_{1} - {XV}_{2}} \right)^{2} + \left( {{YV}_{1} - {YV}_{2}} \right)^{2}}}{TI}} & (7)\end{matrix}$

where TI is the integration time which corresponds to the durationseparating two measurements of distance of the same mobile.

This operation is carried out in the base station of the umbrellamacrocell from the virtual azimuth information provided by directionfinding.

The virtual movement speed information is transmitted to the basestation controller of the macrocell with the aid of an “Afer” interfacewhich has the same characteristics as the standardized “Abis” interface.

The management of the transfers between micro and umbrella cell is thendetermined as a function of the calculated speeds of virtual movement.This management is executed according to steps 11 to 18 of the flowchart of FIG. 7. Upon arrival of a message from a mobile station incommunication, depicted in step 11, a test is carried out in step 12 todetermine whether the mobile station MS is under the control of a basestation of a microcell or of a base station of an umbrella cell. In thefirst case, step 13 checks the number of transfers into or “Handovers”Ho between microcells performed recently by the mobile station. If therate Ho/To obtained during the observation time To is greater than aparameter with maximum value maxHo, then the controller of the basestation BSC of the umbrella cell requests that the transfer be performedto the base station of the umbrella cell. In the contrary case themobile station continues to be managed by the base microstations.

In the second case, step 15 checks that the measured speed of virtualmovement is not greater than a specified threshold value.

If the speed is relatively low, the mobile station is authorized toperform a transfer to a BTS microstation, if it makes a request thereforin step 16.

What is claimed is:
 1. Process for managing an intercellular transfer ofcommunications from a mobile station in a cellular radio communicationsystem including a specific number of microcells inside umbrella cells,wherein each of said microcells and each of said umbrella cells includesa base station for allowing a routing of said communications, comprisingthe steps of: counting a number of handovers H_(o) of said mobilestation between said microcells during a specified observation intervalT_(o) when said mobile station is under control of the base station of amicrocell; calculating a rate of transfer H_(o)/T_(o) for performingsaid intercellular transfer of communications to an umbrella stationwhen said rate of transfer is greater than a specified threshold valuewhen said mobile station is under a control of said base station of saidmicrocell; maintaining said intercellular transfer of communications tosaid base station of said microcell when said rate of transfer is belowsaid specified threshold value when said mobile station is under controlof said base station of said microcell; and performing direction findingto measure a virtual speed of movement of said mobile station inrelation to said base station of said umbrella cell in such a way as toperform said intercellular transfer of communications to said basestation of said microcell when said virtual speed of movement measuredby said direction finding is below a specified speed value when saidmobile station is under control of said umbrella cell.
 2. Processaccording to claim 1, wherein said mobile station communicates with saidbase station in accordance with GSM cellular radio communication networkinterfaces and protocols.
 3. Process according to claim 1, furthercomprising the steps of: performing said direction finding to pinpointat least two successive virtual positions of said mobile station inrelation to said base station of said umbrella cell so as to deducetherefrom said virtual speed of movement of said mobile station in atime interval which separates a pinpointing of said at least twosuccessive virtual positions of said mobile station; comparing saidvirtual speed of movement with said specified speed value so as tomanage said communications of said mobile station at said base stationof said umbrella cell in which said mobile station lies when saidvirtual speed of movement of said mobile station is greater than saidspecified speed value; and comparing said virtual speed of movement withsaid specified speed value so as to transfer management of saidcommunications of said mobile station from said base station of saidumbrella cell in which said mobile station lies to said base station ofsaid microcell when said virtual speed of movement of said mobilestation is less than or equal to said specified speed value.
 4. Processaccording to claim 3, further comprising the step of pinpointing said atleast two successive positions of said mobile station in terms ofazimuth and distance.
 5. Process according to claim 4, wherein azimuthalpinpointing is performed with aid of a multichannel radio directionfinder by determining an angle of arrival of electromagnetic wavestransmitting electromagnetic signals from said mobile station. 6.Process according to claim 5, further comprising the step ofsynchronizing said multichannel radio direction finder to said basestation of said umbrella cell so as to allow angular locating of saidmobile station with a best signal-to-noise ratio.
 7. Process accordingto claim 6, further comprising the step of determining a distanceseparating said mobile station from said base station of said umbrellacell by measuring, in said base station, a time of propagation of radiowaves between said mobile station and said base station.
 8. Processaccording to claim 6, wherein said mobile station communicates with saidbase station in accordance with GSM cellular radio communication networkinterfaces and protocols.
 9. Process according to claim 5, furthercomprising the step of determining a distance separating said mobilestation from said base station of said umbrella cell by measuring, insaid base station, a time of propagation of radio waves between saidmobile station and said base station.
 10. Process according to claim 5,wherein said mobile station communicates with said base station inaccordance with GSM cellular radio communication network interfaces andprotocols.
 11. Process according to claim 4, further comprising the stepof determining a distance separating said mobile station from said basestation of said umbrella cell by measuring, in said base station, a timeof propagation of radio waves between said mobile station and said basestation.
 12. Process according to claim 4, wherein said mobile stationcommunicates with said base station in accordance with GSM cellularradio communication network interfaces and protocols.
 13. Processaccording to claim 3, further comprising the step of determining adistance separating said mobile station from said base station of saidumbrella cell by measuring, in said base station, a time of propagationof radio waves between said mobile station and said base station. 14.Process according to claim 13, wherein said mobile station communicateswith said base station in accordance with GSM cellular radiocommunication network interfaces and protocols.
 15. Process according toclaim 3, wherein said mobile station communicates with said base stationin accordance with GSM cellular radio communication network interfacesand protocols.